"If you decide to withdraw our support and participation in the rating system and begin to release movies without ratings, I will have no choice but to encourage my theatre-owner members to treat unrated movies from the Weinstein Co in the same manner as they treat unrated movies from anyone else," NATO president John Fithian wrote in a letter to the Weinsteins.

He added: "As a father of a nine-year-old child, I am personally grateful that TWC has addressed the important issue of bullying in such a powerful documentary. Yet were the MPAA and NATO to waive the ratings rules whenever we believed that a particular movie had merit, or was somehow more important than other movies, we would no longer be neutral parties applying consistent standards, but rather censors of content based on personal mores."

The Weinsteins hit back with a statement of their own which referenced a recent high-school shooting in suburban Cleveland in which a student who was allegedly the victim of bullying shot dead three other teenagers. "As a company we have the utmost respect for NATO, but to suggest that the film Bully could ever be treated like an NC-17 film is completely unconscionable, not to mention unreasonable," they wrote. "In light of the tragedy that occurred yesterday in Ohio, we feel now is the time for the bullying epidemic to take centre stage, we need to demand our community take action."

While the MPAA ratings are in theory not legally binding, they are firmly established in the US and any breakdown of the current voluntary system would risk the appointment of a more prohibitive federal or state-appointed censor, something neither studios, film-makers or cinema owners would want.